There’s an evil bug in Safari or MoveableType that makes comment spam kind of jump into the actual post itself when you delete it. I’d started ignoring this, since it doesn’t seem to affect the actual post if you don’t save after deleting the comment. Only I guess I saved, because last week one of my posts was indeed replaced by an ad. Luckily Andrew Boaill wrote and told me so I could fix it: thank you Andrew.
Jay Allen promises to release MT-Blacklist, a new, improved, easy-to-install and spamproff anti-comment spam plugin today. This is a good thing.